1. Field of the Invention
The present invention relates to a data line drive circuit which drives a display panel of a matrix type, a liquid crystal display device using the data line drive circuit, and a method for driving data lines.
2. Description of Related Art
In a liquid crystal panel of the liquid crystal display device of a matrix type, the scanning lines and the data lines are extended in a row direction and in a column direction, and pixels are arranged at intersections of the scanning lines and the data lines. Each pixel has an active element (Thin Film Transistor (TFT)). The gate electrode of the active element is connected to the scanning line, and the drain electrode is connected to the data line. Moreover, a liquid crystal capacitance that is equivalent to a capacitive load is connected to the source electrode of the active element, and another side of the liquid crystal capacitance is connected to a common electrode line.
In the liquid crystal display device, in order to drive the scanning lines and the data lines of the liquid crystal panel, a scanning line drive circuit and a data line drive circuit are provided. The scanning line is scanned sequentially from the top to the bottom by the scanning line drive circuit. At this time, a voltage is applied to the liquid crystal capacitance from the data line drive circuit through the active element arranged at each pixel. In the liquid crystal display device, based on the voltage applied to the liquid crystal capacitance, alignment of the liquid crystal molecules changes and the transmissivity of light changes. This change of transmissivity enables color display having grayscale.
In the liquid crystal display device, there is known an alternating current drive method in which a polarity of a voltage (hereinafter referred to as a “pixel voltage”) applied to the liquid crystal capacitance from the data line through the TFT is inverted for every predetermined period. That is, the pixel is driven by an alternating current manner. Here, the polarity means a polarity of the pixel voltage based on a voltage (Vcom) of the common electrode line of the liquid crystal. This is because it is preferable for the pixels to be driven by the alternating current manner, since if a voltage with a fixed polarity is applied to the liquid crystal capacitance, physical characteristics of the liquid crystal molecules will degrade with a lapse of time. As a method for realizing such alternating current driving, there are known the dot inversion drive system where a polarity of the pixel voltage is inverted each time one scanning line is scanned, a two-line dot inversion drive system where a polarity of the pixel voltage is inverted each time two scanning lines are scanned and so on.
Since the voltage applied to the pixel in the inversion drive system is an alternating voltage centering to Vcom, a voltage range for driving is large. These voltages are supplied from the data line drive circuit, and the data line drive circuit consumes a large amount of electric power for driving the liquid crystal display device.
Moreover, along with upsizing of the liquid crystal panel and increasing number of outputs of the data line drive circuit, the data line drive circuit increases its power consumption remarkably.
In a typical data line drive circuit, the liquid crystal panel is driven with all the outputs therefrom being in the same timing. Then, currents concentrate on a same timing and a large current flows instantaneously. In this way, a large EMI (Electro-Magnetic Interference) noise occurs at a moment. In order to reduce this EMI noise, reducing concentration of the currents is needed.
We have now discovered the followings.
As a related art of reducing concentration of currents, a data line drive circuit is described in Japanese Laid-Open Patent Application JP-P 2003-233358A. Referring to FIG. 1, the data line drive circuit is provided with a multi-output amplifier circuit and a delay circuit. The multi-output amplifier circuit is divided into a left amplifier block and a right amplifier block. The operation timings of this data line drive circuit are shown in FIGS. 2A to 2C. When a line output signal shown in FIG. 2A is supplied, the left amplifier block is driven in synchronization with the line output signal as shown in FIG. 2B, and the right amplifier block is driven by a signal obtained by delaying the line output signal in the delay circuit. Thus, by shifting the operation timings of a plurality of amplifier blocks, the concentration of currents can be reduced and the EMI noise can be reduced.
However, since the amplifier blocks execute charging at respective different timings with a fixed time constant, when looking at the amplifier blocks at a certain timing, there is a case where a waveform is fully risen up in the left amplifier block having an early operation timing whereas a waveform is not fully risen up in the right amplifier block having a delayed operation timing. Such a case gives rise to a voltage difference between the right amplifier block and the left amplifier block, and display unevenness occurs as a result. Moreover, recently, there is a panel for a liquid crystal TV using 120-Hz driving. In this liquid crystal display device, since a period when the liquid crystal is charged from the amplifier block decreases to a half of the typical case, a trend of the device to easily generate display unevenness due to the above-mentioned difference of the charging timing becomes more remarkable.
Furthermore, in the liquid crystal display device, there is a case where collection of charges may be conducted in order to curtail power consumption. The collection of charges must be completed before the line output signal falls to a “L” level again after it rose to a “H” level. However, in a related technique, charging is conducted at the different timing and with the fixed time constant. Therefore, there is a case as follows: if the fixed time is secured in order to collect charges, the next period to drive the pixels starts; if the charge collection operation is started earlier, the outputs of the amplifiers may cause electrical shorting through the liquid crystal load. In order to prevent this, the charge collection period must be shortened, and as a result the amount of charges to charge the liquid crystal load increases, which leads to an increase of consumed electric current. This will be contrary to reduction of the EMI noise.
Moreover, an apparatus for driving a liquid crystal is disclosed in Japanese Laid-Open Patent Application JP-A-Heisei 11-85113. Referring to FIG. 3, in this related application, two kinds of switches S1 and S2 that are different in an ON-resistance value are provided at an output of an output circuit. The switches S1 and S2 are switched in response to signals C3 and C4 from the outside and a strobe signal STB. For this reason, even if the control is done with a maximum fineness, the control can be done only for each line, and this application has a same problem as the above-mentioned JP-P 2003-233358A.